The development of microminiature mechanical devices has advanced generally by use of a technique known as micromachining or microfabrication. See for instance, the discussion of microfabrication of mechanical devices by Angell et al. in "Silicon Micromechanical Devices," Scientific American (April 1983), pp. 44-55.
One requirement in the design of a microminiature actuator (hereinafter, microactuator) is that some mechanical actuation means must be provided. A further requirement is that the actuation means must provide sufficient force for reliable actuation. Microactuators designed in the form of microminiature valves may, for example, be employed as gas flow regulators in setting the flow of a carrier gas through a capillary column in a gas chromatograph. The microfabricated valve may be required to open or close a moveable member (typically a moveable membrane, diaphragm, or valve face) against a pressure of 200 pounds per square inch (11 kilograms per square centimeter); to do so, the moving member may be displaced as much as 100 microns. Typically, power from an external power source is provided to the microactuator which employs one of various techniques to convert the applied power to an actuating force. Often the applied power is converted in part or whole to thermal energy, and such microactuators can be considered as being thermally-driven.
A micromachined bi-metallic diaphragm has been employed to provide a thermally-driven actuating force in a microactuator. As the bi-metallic diaphragm is heated, stresses are generated in the structure to deflect the diaphragm, thereby opening or closing the flow of fluid to an attached fluid-bearing system. For example, and with reference to FIGS. 3 and 4 which are reproduced from commonly-assigned U.S. Pat. No. 5,058,856, issued to Gordon et al., a thermally-driven microminiature valve 40 may be actuated from a closed configuration illustrated in FIG. 3, to an open configuration illustrated in FIG. 4. The valve 40 includes a seat substrate 42 which acts as a base; a central flow orifice 44, a lower periphery 45, and a valve seat 46 that surrounds the flow orifice 44. Supported atop the seat substrate 42 is an upper substrate 49 that includes a fixed periphery 47, a central flexible member 50, a lower layer 48 of the flexible member 50, and a boss 43. A nickel layer 51 and an additional serpentine pattern of nickel in a heating element 52 are deposited on a silicon layer 48. Electrical current from an external power source may be conducted through heating elements 52 to generate thermal energy in the form of localized heating, which then conducts through the silicon and nickel layers 48, 51 to cause a temperature increase of approximately 100 degree(s) C. over ambient temperature. The temperature increase causes the valve to open whereupon gas will flow through the flow orifice 44.
However, thermal energy is lost through several paths and in several modes of dissipation. When the valve is closed, thermal energy is conducted from the armature 48 through the boss 43 in the upper substrate to the valve seat 46 and into the bulk of the seat substrate 42. Thermal energy is conducted from the heating pads 52 through the fixed periphery 47 to the seat substrate 42, and gas phase conduction occurs from the lower layer 48 to the seat substrate 42. The thermal energy may flow further into any thermally-conductive structure that is contiguous with the seat substrate 42. The extent of the loss of thermal energy will determine the temperature of the armature; in turn, this temperature (and its rate of change) have a significant effect on the performance of the valve.
Thermally-driven phase change of a fluid has been employed as an actuating force in a microactuator; thermally-driven expansion or contraction of a fluid has also been used as an actuating force. The principal elements of an expansion-contraction design include a cavity formed in a substrate wherein one wall of the cavity is a thin, flexible membrane. The cavity encloses a fixed number of moles of gas or fluid, and when the temperature of the fluid in the cavity is increased, there is a concomitant increase in the pressure-volume (P-V) product of the gas or fluid. The temperature of the cavity may be varied by, for instance, the application of electrical current to a resistive heating element mounted on or inside the cavity, such that the resistive element heats the gas or fluid trapped in the cavity. See, for example, U.S. Pat. No. 4,824,073, issued to Zdeblick.
Irrespective of the type of thermal actuation that is employed in a microactuator, there remains a common need that the thermal energy be efficiently and effectively utilized. Energy that is not efficiently utilized is dissipated from the microactuator in the form of excess heat and as a result the microactuator suffers from unwanted power consumption. Moreover, any portions of the microactuator that are thermally coupled to the thermally-actuated member will accumulate heat. As a result, the microactuator might not actuate as fast as desired, due to the time expended by the thermally-actuated member in dissipating its accumulation of thermal energy when changing from a heated to an unheated (or cooled) state. These problems are especially disadvantageous in microactuators used in fluid flow control applications, such as in pneumatic flow control in gas chromatography, wherein fast actuation is necessary.
Accordingly, there is a need in thermally-actuated microactuators (and especially in microactuators such as are illustrated in FIGS. 3 and 4) for improved thermal isolation of the microactuator with respect to a supporting structure.